I got a sheet of frp (fiber reinforced panel) and put in a shower floor. I made it fairly simple and extended it from the front to the outside edge of the door. The wood sides are ripped to about 3/8th of an inch. They are put on with tiny screws and lot of silicone caulk. The corners and inside edges are also caulked.
Instead of putting the drain in the middle, I put it in the end toward the back of the trailer. I did that because I almost always level my trailer from side to side but keep the front nose up a bubble. I do that because I sleep better with my head up a little. It also makes it predictable on which way water will run for something like this.
The frp bends easily and I intended to recess the drain down about a 1/4 of an inch at a slight slope back to about 4 inches, but I found that the drain I bought doesn't have a strong enough screw system to hold much of anything. Really whimpy!! So until I get a different model it will have to just stay level. The last of the water is still easy to chase out with a squeegee.
Here is the basic floor with the curtain back, but the floor mat still down. (Please excuse the water spots on the camera lens. The camera is a little point and shoot that I use for fishing. It sometimes gets splashed, and I don't realize it. Sorry about that)

And with the shower curtain out. I need to move where my hooks, that the curtain hangs on, are placed so that it is centered and expanded more. That can wait until I get to town to get a few more hooks, as I want to keep the ones on the back wall where I hang it out of the way.

A close up of the mat over the rubber drain plug. Mat isn't straight... I just plopped it down to take the picture.

And this is where things sit traveling.

I am really debating on enclosing my bathroom/shower area with my expanding wall on slides. I like the more open feel of just the curtain on the wall. I may just put a folding bench over the porta-potty. The bench would be good for putting waders on and off. Decisions.... decisions.....
I really like having the floor and drain in front of the door. I can come in to remove my muddy or wet shoes and contain it all in that area. Then I just use my sprayer hose to wash it out the drain. I do have a bucket under the drain to catch the water, but with the sprayer I only use about a 1/2 gal to at most a gal for a shower.
The bumpy texture of the frp does hold on to some dirt so I either brush it while spraying out or give it a finish wipe with a rag. So far it cleans up pretty nice.

I'll have to see what a couple of years of hard use do to it.